encode.php

<?php

function encode($key = 'testkey', $data = 'method=Achievements.leaderboard')
{
	$result = '';
	
	for($i = 0; $i < strlen($data); $i++)
	{
		$char = substr($data, $i, 1);
		$char ^= substr($key, ($i % strlen($key)), 1);
		$result .= $char;
	}
	
	return $result;
}

echo encode('our_secret_key', 'method=blah&some_var=cheese');